This invention relates to a piezoelectric actuator, and more particularly to a piezoelectric actuator which can insure high efficiency in movement conversion even when miniaturized and has improved construction of a vibrating body as well as of a movable body to insure stable rotational movement.
Generally the principle of operation of an ultrasonic motor based on the conventional technology can largely be classified as the standing wave system and the traveling wave system. At first, description is made for an ultrasonic motor based on the standing wave system. FIG. 9 is a perspective explanatory view showing an ultrasonic motor based on the conventional type standing wave system. This ultrasonic motor 900 comprises a vibrating body 910 with a piezoelectric element 912 for generating vibration in the longitudinal direction held between metallic bodies 911, and a movable body 920, and a vibrating piece 913 to be struck against the movable body 920 is provided on and projects from an edge of said vibrating body 910. The vibrating piece 913 is inclined at a specified angle against a direction of the normal line to the movable body 920. When a voltage having a frequency close to a resonance frequency of the vibrating body 910 is loaded, the vibrating body 910 vibrates, and the vibrating piece 913 provided at a tip thereof hits and contacts the movable body 920. As the vibrating piece 913 is inclined at a specified angle against the movable body 920, when the vibrating piece 913 hits the movable body 920, displacement of the vibrating piece 913 in the longitudinal direction is partially converted to that in the lateral direction. Namely, movement of a tip of the vibrating piece follows an elliptical orbit. With this operation, the movable body 920 moves in one direction.
FIG. 10 is an explanatory view showing a principle of operation of another type of ultrasonic motor based on the standing wave system. This ultrasonic motor 1000 makes use of distorted vibration, and has a plurality of projecting sections 1011 provided at a position having a displacement component of movement in the same direction on a vibrating body 1010 (Refer to (c) in the figure). Also, a vertex section of this projecting section 1011 is inclined at a specified angle. The vibrating body 1010 has a piezoelectric element (not shown) having a plurality of electric poles. With this ultrasonic motor 1000, a moving direction of the movable body is decided according to a position where the projection 1010 is provided. When a standing wave is generated by loading a voltage to the piezoelectric element ((a) and (b) in the figure), the projecting section 1011 having a displacement component in one direction as described above contacts a movable body 1020 ((c) in the figure). For this reason, the movable body 1020 moves in one direction described above. The movable body 1020 can be moved in the opposite direction by changing a position of the projecting section 1011 ((c) in the figure).
Next, description is made for an ultrasonic motor based on the traveling wave system. FIG. 11 is a perspective view showing an ultrasonic motor 1100 based on the conventional type of traveling wave system. The ultrasonic motor 1100 comprises a vibrating body 1110 comprised of a ring-shaped metallic body 1102 with a plurality of projecting sections 1101 formed in the peripheral direction and a piezoelectric ceramic element 1103 adhered to a bottom surface of the metallic body 1102, and a ring-shaped movable body 1120 pressure-fitted to a surface of this vibrating body 1110. A frictional member with excellent wearing-resistance is spanned over a contact section 1121 of the movable body 1120. Also, as shown in FIG. 12, a plurality of driving electrodes 1104 corresponding to the projecting sections 1101 are provided in the piezoelectric ceramics 1103. This ring-shaped ultrasonic motor 1100 is used as a lens actuator for automatically focusing, for instance, a single-lens reflex camera.
In this ultrasonic motor 1100, distorted vibration is loaded to the vibrating body 1110 by controlling an amplitude of the polarized piezoelectric ceramic element 1103, and exciting a traveling wave of the distorted vibration according to a phase difference of the loaded voltage. The distorted vibration of the traveling wave converts the movement in the vertical direction to movement in the lateral direction according to a thickness of the vibrating body 1110, and gives an elliptical movement as shown in FIG. 13 to the projecting section 1101 of the vibrating body 1110. The movable body 1120 is driven by the frictional force with the projecting section 1101 and moves. As height of the projecting section 1101 gives influence to displacement in the lateral direction, it is required to set a moving speed of the movable body 1120.
Further as shown in FIG. 14, the ultrasonic motor based on the traveling wave may have a disk-shaped configuration. This ultrasonic motor 1200 has a vibrating body 1210 comprising a disk-shaped metallic body 1203 with a plurality of projecting sections 1201 formed in the peripheral direction, and a shaft holder 1202 located in the center, a piezoelectric element 1204 adhered to a bottom surface of the metallic body 1203, and a disk-shaped movable body 1220 press-fitted to a surface of this vibrating body 1210. The remaining configuration and the operation are the same as those of the ring-shaped ultrasonic motor 1100 shown in FIG. 14, so that description thereof is omitted herein.
In the ultrasonic motors 900, 1000, 1100, and 1200 based on the conventional technology as described above, the rotationally moving capability is dependent on the bending rigidity of the vibrating body, the mass of the projecting section, and the mechanical machining precision and homogeneity of a tip of the projecting section contacting the movable body as well as on friction with a contacting surface of the movable body, so that it is difficult to improve and stabilize the movement conversion efficiency. For instance, in the disk-shaped ultrasonic motor 1100 based on the traveling wave system, there have been problems that the metallic body 1102 is hardly distorted because the metallic body 1102 is disk-shaped, the movement conversion efficiency is low and the rotation is sometimes unstable because the mass of the projecting section 1101 is large and, as a result, the projecting section can hardly be vibrated.
Also, if a diameter of the ultrasonic motor is made smaller, an oscillatory wave generated by the vibrating body 1110 has no vibration node, so that the oscillatory wave is easily attenuated as the diameter is made smaller. For this reason, the efficiency in conversion of movement from the piezoelectric element 1103 to the movable body 1120 becomes remarkably low, which makes it extremely difficult to reduce size of the ultrasonic motor. Also as the ultrasonic motor becomes smaller, influence of bending rigidity of the vibrating body 1110 becomes larger, so that miniaturization of ultrasonic motor is limited.